The present invention relates to a method of making a substantially homogenous, silicon clathrate composition, and more particularly, to a method of making a substantially homogenous, alkali-metal free Si136 clathrate composition.
Thermoelectric materials are solid-state materials that can reversibly convert electricity to heat. Key properties for any thermoelectric material are thermoelectric power, electrical conductivity and thermal conductivity. These three parameters are often combined into a dimensionless figure of merit ZT that characterizes efficiency of conversion of thermal energy into electrical energy (or vice versa) (see, e.g. Mahan, G., Sales, B., and Sharp, J., Physics Today, p. 42, (March 1997). Most thermoelectrical materials used presently have ZTxcx9c1. However, for greater efficiency, thermoelectric materials with a ZT value approaching 2 or greater are preferred. In order to meet these requirements, thermoelectric materials must exhibit large thermoelectric powers and electrical conductivities while at the same minimizing thermal conductivities. One attempt to synthesize thermoelectric materials with larger ZT values has been the development of semiconductor clathrates (i.e., metal-doped semiconductors).
Semiconductor clathrates such as alkali-metal silicon clathrates having the formula MxSi136 with xxcx9c3 to 6 have been found to exhibit large thermoelectric powers. As a result, MxSi136 clathrates appear promising as new thermoelectric materials. Unfortunately, current methods of synthesizing these clathrates by thermal decomposition of alkali-metal silicides produce a mixture of MxSi136 and MySi46 phases (see, e.g., Cross, C., Pouchard, M., and Hagenmuller, P., C. R. Acad. Sc. Paris, vol. 260, p. 4764 (1965); Cross, C., Pouchard, M., and Hagenmuller, P., J. Solid State Chem. Phys., vol. 2, p. 570, (1970); Roy, S. B., Sim., K. E., and Caplin, A. D., Phil. Mag. B, vol. 65, p. 1445 (1992)). As known to those skilled in the art, MySi46 has metallic character. As a result, mixtures of MxSi136 and MySi46 phases exhibit significantly lower thermoelectric powers than homogenous MxSi136 phases. To overcome this deficiency, extensive purification steps are required to obtain a homogenous MxSi136 sample which increases the cost of production. Therefore, there is a need in the art for a simple and efficient method of synthesizing homogenous MxSi136 clathrates, which will facilitate large scale commercial applications.
Accordingly, it is an object of the present invention is to provide a simple and effective of making homogenous MxSi136 clathrates. It also an object of the present invention is to provide a method of making homogenous MxSi136 clathrates that will allow the number of alkali-metal atoms per unit crystallographic cell xe2x80x9cxxe2x80x9d to be varied from a maximum value 24 to as low as 0.
The present invention provides a simple and effective method of making a substantially homogenous silicon clathrate composition. In one embodiment, the method entails rapidly heating, under a vacuum, an alkali-metal silicide to a decomposition temperature of at least 365xc2x0 C. thereby forming a silicon clathrate having the formula MxSi136 where M is the alkali-metal and 3xe2x89xa6xxe2x89xa624. Preferably, the silicide is heated to the decomposition temperature within 2 minutes, and more preferably within 1 minute. Preferably, the alkali silicide is heated for at least 30 minutes with at least 60 minutes being more preferred. Examples of alkali-metals to be used for the alkali-metal suicides are sodium, potassium, cesium, rubidium and mixtures thereof. Heating is preferably conducted under a vacuum of at least 10xe2x88x925 Torr, with at least 10xe2x88x926 Torr being more preferred.
Advantageously, the method provides a silicon clathrate composition where at least 90 weight percent of the silicon clathrate present in the composition is MxSi136, with at least 95 weight percent MxSi136 being more preferred and at least 98 weight percent MxSi136 being more preferable. Preferably, the silicon clathrate formed by the above-described method has 3xe2x89xa6xxe2x89xa66.
In another embodiment, the present invention provides a method of reducing the alkali-metal content of an alkali-metal silicon clathrate having the formula MxSi136 where M is the alkali-metal and x greater than 0, with x greater than 3 being more preferred. This method entails a two-step process of: (a) treating the silicon clathrate with an acid, preferably strong acid; and (b) rapidly heating, under a vacuum, the treated silicon clathrate to a decomposition temperature of at least 400xc2x0 C. The two steps are repeated, as necessary, to reduce the alkali-metal content to a desired number of atoms. Acids to be used include sulfuric acid, hydrochloric acid, hydrofluoric acid, nitric acid and mixtures thereof. If desired the alkali-metal content xe2x80x9cxxe2x80x9d can be reduced to zero to provide an alkali-metal free silicon clathrate having the formula Si136.